Doubly-fed induction machines are used in connection with wind power stations located alone or in wind parks. In principle the doubly-fed induction machine is also used in water power stations. Due to the demand to use wind parks with a weak connection to the grid, for example in the case of an off-shore wind park, unbalanced grid operation of the wind power station in the case of a grid fault has to be taken into consideration in construction of control units of the doubly-fed induction machines. One aim is to avoid disconnection of the wind power stations from the grid for example in the case of drop of the grid voltage. In order to avoid damages in the control units of a wind power station or a damage of the doubly-fed induction machine, a so-called “crowbar” is used, which is connected to the rotor side of the doubly-fed induction machine in order to protect the machine side converter. Crowbar circuits can be of different designs and are well known. This enables the wind power station to ride through grid faults without being disconnected due to over-voltages. Additionally, in order to avoid over-voltages in the DC-link, a DC-chopper can be equipped to reduce the DC-link voltage. In common control units, the converter controller protects the doubly-fed induction machine by controlling the operation of the crowbar as well as the DC-chopper in the DC-link. However, using the converter controller has the disadvantage that the sampling rate to control the operation of DC-chopper and the crowbar cannot be increased without low effort, because the converter controller also controls the machine side and the line side converter.
Furthermore it is known from the European patent application EP 1 965 075 A1 to control the crowbar via a separate device. But, a separate device controlling merely the operation of a crowbar is not suitable to achieve the desired stability during grid faults.
Therefore, it is an object of the present invention to provide a protection system and a protection method of a doubly-fed induction machine which can be easily used with high sampling rates and which provides a good protection for the doubly fed induction machine. Furthermore, it is an object of the present invention to propose an advantageous use of the inventive protection system.